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Stratigraphic and sedimentological observations from seismic data across the Chicxulub impact basin

C. BELL, J. V. MORGAN, G. J. HAMPSON, B. TRUDGILL

Abstract

Seismic data across the offshore half of the Chicxulub impact crater reveal a 145 kmdiameter post-impact basin to be a thickening of Tertiary sediment, which thickens by ~0.7 sec from the basin margin to the basin center. The basin existed long after the impact and was gradually infilled to its current flat surface. A suite of seismic horizons within the impact basin have been picked on four reflection lines across the crater. They reveal that the western and northwestern parts of the impact basin were filled first. Subsequently, there was a dramatic change in the depositional environment, indicated by an unconformable surface that can be mapped across the entire basin. A prograding shelf sequence downlaps onto this unconformity in the eastern basin. The seismic stratigraphic relationships suggest a marine regression, with sedimentation becoming gradually more passive as sediments fill the eastern part of the impact basin. The central and northeastern parts of the basin are filled last. The onshore hole Yaxcopoil-1 (Yax-1), which was drilled on the flanks of the southern basin, has been projected onto the offshore seismic data to the west of the crater center. Using dates obtained from this onshore well and regional data, approximate ages have been placed on the most significant horizons in the offshore seismic data. Our preliminary interpretation is that the western and northwestern basins were almost entirely filled by 40 Ma and that the marine regression observed in the eastern basin is early Miocene in age. Offshore seismic stratigraphic analyses and onshore data within Yax-1 suggest that the early Paleocene is highly attenuated across the impact basin. The Mesozoic section appears to be ~1 km thicker offshore than onshore. We calculate that, given this offshore thickening, the volume of Mesozoic rocks that have been excavated, melted, or vaporized during impact is around 15% larger than expected from calculations that assume the offshore thickness is equal to that onshore. This has significant consequences for any environmental calculations. The current offset between the K-T boundary outside and inside the crater is ~700 m. However, infilling of basins with sediments is usually accompanied by subsidence, and immediately following the impact, the difference would have been smaller. We calculate the original topographic offset on the K-T boundary to have been between 450 and 700 m, which is in agreement with depthdiameter scaling laws for a mixed target.